Heat conductive sheet and method for producing heat conductive sheet

ABSTRACT

A heat conductive sheet having excellent adhesion between an acrylic resin layer and a supporting sheet is provided. The heat conductive sheet includes a heat conductive resin layer including a heat conductive acrylic resin composition; and a supporting resin layer (supporting sheet) containing a polyvinyl acetal resin and a styrene-vinyl isoprene block copolymer. Crosslinking of the supporting sheet with acrylic monomers of the acrylic heat conductive resin layer enables improvements in adhesion between the heat conductive resin layer and the supporting sheet.

TECHNICAL FIELD

The present disclosure relates to an acrylic heat conductive sheet usedas a heat dissipation measure for an electronic component among othersand a method for producing a heat conductive sheet. This applicationclaims priority to Japanese Patent Application No. 2014-74776 filed onMar. 31, 2014, the entire contents of which are hereby incorporated byreference.

BACKGROUND ART

In recent years, heat conductive sheets have been used as a heatdissipation measure for electronic components (e.g. PLT. 1). In heatconductive sheets, in order to improve adhesion to an electroniccomponent, heat conductive resin layers having excellent flexibilityhave been employed; however, occurrences of stretching and breakageduring use of heat conductive resin layers having excellent flexibilityhave led to difficulties in handling. Therefore, a supporting sheet forreinforcing the heat conductive resin layer has been employed.

However, for example, in the case of using a PET film as the supportingsheet, an acrylic heat conductive resin layer would easily peel away.Priming the PET film is thus necessary, increasing the number ofprocesses.

PRIOR ART LITERATURE Patent Literatures

PLT 1: Japanese Unexamined Patent Application Publication No.2007-123624

SUMMARY OF THE INVENTION Problem to Be Solved by the Invention

In view of such conventional circumstances, the present disclosureprovides a heat conductive sheet having excellent adhesion between anacrylic resin layer and a supporting sheet and a method for producingthe same.

Solution to Problem

As a result of earnest investigation, the present inventors have foundthat adhesion properties between an acrylic resin layer and a supportingsheet can be improved by using a supporting sheet containing a polyvinylacetal resin and a styrene-vinyl isoprene block copolymer.

Thus, a heat conductive sheet according to the present disclosurecomprises a heat conductive resin layer comprising a heat conductiveacrylic resin composition and a supporting resin layer containing apolyvinyl acetal resin and a styrene-vinyl isoprene block copolymer.

Furthermore, a supporting sheet according to the present disclosurecomprises a polyvinyl acetal resin and a styrene-vinyl isoprene blockcopolymer.

Still further, a method for manufacturing a heat conductive sheetaccording to the present disclosure comprises the steps of: forming asupporting resin layer by mixing and sheet-forming a polyvinyl acetalresin and a styrene-vinyl isoprene block copolymer; and forming a heatconductive resin layer by curing a heat conductive acrylic resincomposition in a state of contact with the supporting resin layer.

Yet further, a method for manufacturing a supporting sheet according tothe present disclosure comprises mixing and sheet-forming a polyvinylacetal resin and a styrene-vinyl isoprene block copolymer.

Advantageous Effects of Invention

According to the present disclosure, crosslinking of the polyvinylacetal resin and the styrene-vinyl isoprene block copolymer contained bythe supporting resin with acrylic monomers of the acrylic resin layerenables improvements in adhesion between the acrylic resin layer and thesupporting sheet.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross sectional view illustrating one example of a heatconductive sheet according to one embodiment of the present disclosure.

FIG. 2 is an explanatory diagram of a T-type peel strength measurementmethod of a heat conductive resin layer and a supporting resin layer.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present disclosure will now be more particularlydescribed according to the following order with reference to theaccompanying drawings.

1. Heat Conductive Sheet 2. Heat Conductive Sheet Manufacturing Method3. Examples

1. Heat Conductive Sheet

A heat conductive sheet according to the present disclosure has a heatconductive resin layer comprising a heat conductive acrylic resincomposition and a supporting resin layer containing a polyvinyl acetalresin and a styrene-vinyl isoprene block copolymer.

Preferable thickness of the heat conductive sheet is from 0.1 to 10.0 mmand preferable thickness of the supporting resin layer is 0.001 to 0.500mm.

FIG. 1 is a cross sectional view illustrating one example of a heatconductive sheet according to one embodiment of the present disclosure.This heat conductive sheet has a heat conductive resin layer 11 formedby curing a heat conductive acrylic resin composition and a supportingresin layer 12 for supporting the heat conductive resin layer 11.Furthermore, a release-treated film 13 which peels away at utilizationtime is applied to the surface of the heat conductive resin layer 11.

The heat conductive resin layer 11 is formed by curing the heatconductive acrylic resin composition. Thermal conductivity of the heatconductive resin layer 11 is preferably 0.3 W/m·K or more. Furthermore,compressibility for the heat conductive resin layer 11 is preferablyfrom 1% to 80% when under a load of 1 kgf/cm². As compressibilityincreases, flexibility becomes excellent and excellent adhesionproperties to heat-generating elements and heat-dissipating elements areobtainable. Furthermore, hardness for the heat conductive resin layer 11as measured on an Asker durometer type C scale (JISK 7312) is preferablyfrom 5 to 60.

As the heat conductive acrylic resin composition, conventionally knownheat conductive acrylic resin compositions may be used. Examples of heatconductive acrylic resin compositions include those containing (A) amonofunctional (meth)acrylate, (B) a polyfunctional (meth)acrylate, (C)a photopolymerization initiator, (D) heat conductive particles, (E) aplasticizer and (F) a thiol compound.

Monofunctional (meth)acrylate including alkyl (meth)acrylate having astraight-chain or branched-chain alkyl group is preferably used.Examples of alkyl (meth)acrylate include methyl (meth)acrylate, ethyl(meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate,n-butyl (meth)acrylate, sec-butyl (meth)acrylate, tert-butyl(meth)acrylate, isobutyl (meth)acrylate, n-pentyl (meth)acrylate,isopentyl (meth)acrylate, n-hexyl (meth)acrylate, cyclohexyl acrylate,n-heptyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, n-nonyl (meth)acrylate,isononyl (meth)acrylate, n-decyl (meth)acrylate, isodecyl(meth)acrylate, n-dodecyl (meth)acrylate, isomyristyl (meth)acrylate,n-tridecyl (meth)acrylate, n-tetradecyl (meth)acrylate, n-stearyl(meth)acrylate, isostearyl (meth)acrylate and n-lauryl (meth)acrylate,among others; these may be used individually or in a combination of twoor more

Examples of polyfunctional (meth)acrylates include bifunctional(meth)acrylates such as 1,3-butanediol di(meth)acrylate, 1,4-butanedioldi(meth)acrylate, neopentanediol di(meth)acrylate, 1,6-hexanedioldi(meth)acrylate, 1,9-nonanediol di(meth)acrylate, ethylene glycoldi(meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycoldi(meth)acrylate, tripropylene glycol di(meth)acrylate and polypropyleneglycol di(meth)acrylate and (meth)acrylates having three or morefunctional groups such as trimethylolpropane tri(meth)acrylate,pentaerythritol tri(meth)acrylate, tris(acryloyloxyethyl) isocyanurate,ditrimethylolpropane tetra(meth)acrylate, pentaerythritoltetra(meth)acrylate, dipentaerythritol monohydroxy penta(meth)acrylateand dipentaerythritol hexa(meth)acrylate, among others; these may beused individually or in a combination of two or more.

Examples of the photopolymerization initiator include benzophenone,benzoin, benzoin alkyl ether, benzyl dimethyl ketal, α-hydroxy ketoneand acylphosphine oxide based types, among others; these may be usedindividually or in a combination of two or more.

Examples of the heat conductive particles include metal hydroxides suchas aluminum hydroxide and magnesium hydroxide, metals such as aluminum,copper and silver, metal oxides such as those of alumina and magnesia,nitrides such as aluminum nitride, boron nitride and silicon nitride,and carbon nanotubes, among others; these may be used individually or ina combination of two or more.

Content of the heat conductive particles in the heat conductive acrylicresin composition is preferably 100 to 2,000 pts. mass and morepreferably 300 to 650 pts. mass with respect to 100 pts. mass ofmonofunctional (meth)acrylate. Insuficient content of the heatconductive particles results in difficulties in achieving sufficientlyhigh thermal conductivity in the heat conductive sheet; excessivecontent of the heat conductive particles tends to decrease flexibilityof the heat conductive sheet. Furthermore, in the case of using two heatconductive fillers having differing average particle diameters, thesmall-diameter filler and the large-diameter filler are preferablyblended at a ratio of 15:85 to 90:10.

As the plasticizer, for example, at least one of adipic acid esters,pimelic acid esters, suberic acid esters, azelaic acid esters andsebacic acid esters may be selected for use. Such a dicarboxylic acidester can be obtained by conventional esterification of an alcohol and adicarboxylic acid selected from adipic acid (HOOC—(CH₂)₄—COOH), pimelicacid (HOOC—(CH₂)₅—COOH), suberic acid (HOOC—(CH₂)₆—COOH), azelaic acid(HOOC—(CH₂)₇—COOH), and sebacic acid (HOOC—(CH₂)₈—COOH). Moreparticularly, as the plasticizer, it is preferable to use at least oneof diisodecyl adipate, diisodecyl pimelate, diisodecyl suberate,diisodecyl azelate and diisodecyl sebacate being esterified fromisodecyl alcohol and one of adipic acid, pimelic acid, suberic acid,azelaic acid.

As the thiol compound, polyfunctional thiol having two or morefunctional groups may be used. Usable examples of polyfunctional thiolhaving two or more groups include 3-functional thiol compounds such as1,3,5-tris(3-mercapto-butyloxyethyl)-1,3,5-triazine-2,4,6(1H, 3H,5H)-trione, 2-methyl-2-((3-mercapto-1-oxopropyl)-methyl)propane-1,3-diylbis(3-mercaptopropionate), trimethylolpropanetristhiopropionate, trimethylolpropane tristhioglycolate and2,4,6-trimercapto-s-triazine, 4-functional thiol compounds such aspentaerythritol tetrakis(3-mercapto butyrate), pentaerythritol tetrakisthioglycolate, dipentaerythritol tetrakis thioglycolate, pentaerythritoltetrakis thiopropionate, 5-functional thiol compounds such asdipentaerythritol pentakis thioglycolate and 6-functional thiolcompounds such as dipentaerythritol hexakis thioglycolate; these may beused individually or in a combination of two or more.

Furthermore, other components such as antioxidants, thermal degradationinhibitors, flame retardants and colorants may be blended into the heatconductive acrylic resin composition.

Examples of the antioxidants include primary antioxidants which captureradicals generated by thermal degradation and secondary antioxidantswhich decompose peroxides generated by thermal degradation; these may beused individually or in a combination of two or more. Examples ofthermal degradation inhibitors include acrylic acid monoesters such as1,1-bis(2-hydroxy-3,5-di-tert-pentylphenypmethane.

Moreover, the heat conductive acrylic resin composition is not limitedto the configuration described above and other heat conductive acrylicresin compositions may be used. For example, commercially availableproducts include GNS of the TransCool series of INOAC CORPORATION andShirokinon Series (TAICA CORPORATION) and 5580H (Sumitomo 3M Ltd.),among others.

The supporting resin layer 12 is a supporting sheet containing apolyvinyl acetal resin and a styrene-vinyl isoprene block copolymer.

The polyvinyl acetal resin is obtained by acetalization of an aldehydewith a polyvinyl alcohol. Polyvinyl alcohols are, for example, obtainedby saponification of polyvinyl acetate; degree of saponification isgenerally 80 mol % to 99.8 mol %. Polymerization degree of the polyvinylalcohol is generally 200 to 3,000. Examples of aldehyde includeformaldehyde, acetaldehyde, n-butyraldehyde, isobutyraldehyde, n-hexylaldehyde and n-valeraldehyde, among others; these may be usedindividually or in a combination of two or more. Among these, reactingacetaldehyde or butyraldehyde with the polyvinyl alcohol to obtainpolyvinyl acetal or polyvinyl butyral for use is particularlypreferable.

Molecular weight of the polyvinyl acetal resin is 5×10⁴ to 20×10⁴ andmore preferably 8×10⁴ to 15×10⁴. An excessively low molecular weightcauses exhibition of adhesiveness and might degrade dry properties ofthe supporting sheet, and an excessively high molecular weight mightdegrade flexibility of the supporting sheet.

Furthermore, glass transition temperature (Tg) of the polyvinyl acetalresin is 50 to 150° C. and more preferably 80 to 120° C. An excessivelylow glass transition temperature causes exhibition of adhesiveness andmight degrade drying properties of the supporting sheet, and anexcessively high glass transition temperature might degrade flexibilityof the supporting sheet.

Furthermore, degree of acetalization of the polyvinyl acetal resin(degree of butyralization in the case of polyvinyl butyral resin) is 50to 90 mol % and more preferably 60 to 80 mol %. An excessively lowdegree of acetalization might degrade compatibility with thestyrene-vinyl isoprene block copolymer; an excessively high degree ofacetalization might degrade thermal tolerance of the supporting sheet.

Degree of acetalization is a mole fraction expressed as a percent (mol%) obtained by dividing the amount of ethylene groups to which acetalgroups are bonded by the total amount of ethylene groups in main chains.The acetalization degree can be obtained in accordance with JIS K6728“Test Methods for Polyvinyl Butyral” by measuring the degree ofacetylation (amount of acetyl groups) and the hydroxyl group content(the amount of vinyl alcohol), calculating the molar fractions based onthe measurement results and subtracting the molar fractions of theacetyl groups and hydroxyl groups from 100 mol %.

Commercially available polyvinyl acetal resins include, for example,S-LEC BX-1, BX-5, KS-3 and KS-5 (SEKISUI CHEMICAL CO., LTD.), amongothers.

The styrene-vinyl isoprene block copolymer is a triblock copolymercomprising a styrene block and a vinyl-polyisoprene block and is athermoplastic elastomer in which the styrene units are hard segments andthe vinyl-polyisoprene units are soft segments.

Styrene content of the styrene-vinyl isoprene block copolymer is 10% to30% and preferably 15% to 25%. Excessively low styrene content leads toinsufficient hard segments which might degrade rubber elasticity;excessively high styrene content might degrade flexibility of thesupporting sheet.

Furthermore, a glass transition temperature (Tg) of the styrene-vinylisoprene block copolymer is −40 to +40° C. and preferably −20 to +20° C.An excessively low glass transition temperature causes exhibition ofadhesiveness and might degrade drying properties of the supportingsheet, and an excessively high glass transition temperature mightdegrade flexibility of the supporting sheet.

Examples of commercially available styrene-vinyl isoprene blockcopolymers include HYBRAR 5125 and 5127 (KURARAY CO., LTD.), amongothers.

Furthermore, the polyvinyl acetal and the styrene-vinyl isoprene blockcopolymer are preferably blended at 9:1 to 7:3 mass ratio. Aninsufficient blending amount of the styrene-vinyl isoprene blockcopolymer leads to adhesion properties with the heat conductive resinlayer being unobtainable; an excessive blending amount of thestyrene-vinyl isoprene block copolymer might degrade drying propertiesand/or strength of the supporting sheet.

Furthermore, a cross-linking agent may be added to the supporting resinlayer 12. Examples of usable cross-linking agents include isocyanatecrosslinking agents, epoxy crosslinking agents and silicone-basedcrosslinking agents, which may be used individually or in a combinationof two or more.

The surface of the supporting resin layer 12 is preferably rough. Arough surface of the supporting resin layer 12 increases contact areawith the heat conductive resin, leading to an increase in vinyl groupcross-linking points, thereby improving adhesion due to this anchoringeffect.

As the release-treated film 13, a product obtained by coating a releaseagent such as silicone to PET (polyethylene terephthalate), OPP(oriented polypropylene), PMP (poly-4-methylpentene-1) or PTFE(polytetrafluoroethylene), among others.

In such a heat conductive sheet, crosslinking of the supporting resinlayer containing polyvinyl acetal resin and styrene-vinyl isoprene blockcopolymer with acrylic monomers of the acrylic heat conductive resinlayer enables improvements in adhesion between the heat conductive resinlayer and the supporting sheet. Additionally, because the surface of thesupporting sheet is dry, improvements in workability are enabled.Furthermore, due to having excellent thermal conductivity andflexibility, the heat conductive sheet is suitable for use in precisionequipment such as hard disk devices and laser devices.

It should be noted that in the embodiment described above, aconfiguration was described in which the supporting resin layer wasprovided on one side of the heat conductive resin layer; however,configurations in which the supporting resin layers are provided on bothsides of the heat conductive resin layer and configurations in which theheat conductive resin layers are provided on both sides of thesupporting resin layer are possible. Furthermore, in the embodimentdescribed above, the supporting sheet was used in a heat conductivesheet; however, the present disclosure is not limited thereto. Forexample, the supporting sheet may be used as a support material for anacrylic-type adhesive tape or bonding tape.

2. Heat Conductive Sheet Manufacturing Method

The method for manufacturing a heat conductive sheet according to thepresent disclosure comprises the steps of forming a supporting resinlayer by mixing a polyvinyl acetal resin and a styrene-vinyl isopreneblock copolymer which is then sheet-formed, and forming a heatconductive resin layer by curing a heat conductive acrylic resincomposition in a state of contact with the supporting resin layer.

In the step of forming the supporting resin layer, 10 to 65 pts. massand preferably 10 to 45 pts. mass of the styrene-vinyl isoprene blockcopolymer with respect to 100 pts. mass of the polyvinyl acetal resin isadded and mixed to prepare a supporting resin composition. In sheetforming, for example, the supporting resin composition is uniformlyapplied to a release-treated film using a bar coater and dried. Thepolyvinyl acetal resin and the styrene-vinyl isoprene block copolymerare thermoplastic resins which therefore can be sheet-formed without asolvent by using an injection molding method, extrusion molding methodor kneading method, among others.

In the step of forming the heat conductive resin layer, the heatconductive acrylic resin composition is cured in a state of contact withthe supporting resin layer.

Presumably, by vinyl group components of the styrene-vinyl isopreneblock copolymer of the supporting resin layer and acrylic monomercomponents of the heat conductive acrylic resin composition being phasecompatible or by curing, mutual integration occurs enabling improvementsin adhesion. Usable curing methods include conventional curing methodssuch as by light, heat and solvent; however, it is preferable to use atleast one of light and heat.

Furthermore, in the heat conductive resin layer forming step, the heatconductive acrylic resin composition may be cured in a state of beingsandwiched between two of the supporting sheets and may be cured in astate of being applied to both sides of the supporting sheet.

In such a method for manufacturing a heat conductive sheet, in the stepof forming the heat conductive resin layer, the heat conductive resinlayer and the supporting resin layer can be strongly bonded by curingthe heat conductive acrylic resin composition.

EXAMPLES 3. Examples

Examples of the present disclosure will now be described. In thefollowing examples, supporting resin layers were formed by mixingpolyvinyl acetal and styrene-vinyl isoprene block copolymer atpredetermined mass ratios. Next, heat conductive resin layers wereformed by curing heat conductive acrylic resin compositions on thesupporting resin layers to manufacture heat conductive sheets.Subsequently, for each of the heat conductive sheets, tensile strength,T-type peel strength of the supporting resin layer and the heatconductive resin layer and tack of the surface of the supporting resinlayer were measured/evaluated. It should be noted that the presentinvention is not limited to these examples.

Manufacturing of the heat conductive sheets, tensile strengthmeasurement/evaluation, T-type peel strength measurement/evaluation, andtack measurement/evaluation were performed in the following manner.

Heat Conductive Sheet Manufacturing

Polyvinyl acetal (S-LEC BX-1, SEKISUI CHEMICAL CO., LTD.) andstyrene-vinyl isoprene block copolymer (HYBRAR 5125, KURARAY CO., LTD.)were mixed at predetermined mass ratios to obtain supporting resincompositions. These supporting resin compositions were applied torelease-treated films comprising PET using a bar coater at a thicknessof 0.01 mm to form supporting resin layers.

Furthermore, 100 pts. mass 2-ethylhexyl acrylate as a monofunctionalacrylate, 47 pts. mass castor-oil derivative fatty acid ester as aplasticizer, 1.5 pts. mass hydroxy pivalic acid neopentyl glycoldiacrylate (KAYARAD FM-400, Nippon Kayaku Co., Ltd.) as a polyfunctionalacrylate, 1.4 pts. mass photopolymerization initiator (IRGACURE 819,BASF) and 400 pts. mass aluminum hydroxide powder (80 pan averageparticle diameter) along with 400 pts. mass aluminum hydroxide powder (8μm average particle diameter) as heat conducting fillers were mixed toprepare heat conductive acrylic resin compositions.

The heat conductive acrylic resin composition was applied onto thesupporting resin layers before irradiating ultraviolet light at anintensity of 1 mW/cm² for five minutes simultaneously to both sides ofthe heat conductive acrylic resin composition surface and the supportingresin layer surface to manufacture heat conductive sheets comprising0.01 mm thick supporting resin layers and 1.0 mm thick heat conductiveresin layers.

Tensile Strength Measurement/Evaluation

Samples of the heat conductive sheet comprising the heat conductiveresin layer 11 and the supporting resin layer 12 were manufactured byusing a hand press and a punch die (dumbbell-shaped No. 3) having anarrow section which was 20 mm long and 5 mm wide. A tensile testingmachine (RTG-1225, ORIENTEC CORPORATION) pulling at a speed of 500mm/min was used to measure strength (tensile load divided bycross-sectional area of the samples) at the time of severance(breakage). A tensile strength of 0.4 MPa or more was evaluated as“good” (hereinafter referred to as “G”) and a tensile strength of lessthan 0.4 MPa was evaluated as “fail” (hereinafter referred to as “F”).

T-type peel strength Measurement/Evaluation

As shown in FIG. 2, a tensile testing machine (RTG-1225, ORIENTECCORPORATION) was used in a T-peel test to measure delamination strengthof the heat conductive resin layer 11 and the supporting resin layer 12in a state in which a release-treated film 13A comprising PET was pastedto the heat conductive resin layer 11 and a release-treated film 13Bcomprising PET was pasted to the supporting resin layer 12. Peel speedin this case was 300 mm/min and sample width was 15 mm. Delaminationstrengths of 0.2 N/20 mm or more were evaluated as G, less than 0.2 butmore than 0.15 N/20 mm were evaluated as intermediate (hereinafterreferred to as “I”), and less than 0.15 N/20 mm were evaluated as F.

Surface Tack Measurement/Evaluation

A TAC-II tack tester manufactured by RHESCA CO., LTD. was used as a tacktesting device to measure probe tack when pressing and peeling away; tothe supporting resin layer, an unheated aluminum cylinder with a 10 mmdiameter approached at 30 mm/min, was pressed for 5 seconds with a 196 gload and was peeled away at a speed of 120 mm/min over a distance of 5mm. Supporting resin layer tack was evaluated as G for less than 10kN/m², I for 10 kN/m² or more but less than 15 kN/m², and F for 15 kN/m²or more.

Comparative Example 1

Other than that polyvinyl acetal resin and styrene-vinyl isoprene blockcopolymer were mixed at a mass ratio of 10:0 to form the supportingresin layer, a heat conductive sheet was manufactured as describedabove. As represented in Table 1, Comparative Example 1 had a tensilestrength evaluation of G, a T-type peel strength evaluation of F, and asurface tack evaluation of G.

Example 1

Other than that polyvinyl acetal resin and styrene-vinyl isoprene blockcopolymer were mixed at a mass ratio of 9:1 to form the supporting resinlayer, a heat conductive sheet was manufactured as described above. Asrepresented in Table 1, Example 1 had a tensile strength evaluation ofG, a T-type peel strength evaluation of I, and a surface tack evaluationof G.

Example 2

Other than that polyvinyl acetal resin and styrene-vinyl isoprene blockcopolymer were mixed at a mass ratio of 8.5:1.5 to form the supportingresin layer, a heat conductive sheet was manufactured as describedabove. As represented in Table 1, Example 2 had a tensile strengthevaluation of G, a T-type peel strength evaluation of G, and a surfacetack evaluation of G.

Example 3

Other than that polyvinyl acetal resin and styrene-vinyl isoprene blockcopolymer were mixed at a mass ratio of 8:2 to form the supporting resinlayer, a heat conductive sheet was manufactured as described above. Asrepresented in Table 1, Example 3 had a tensile strength evaluation ofG, a T-type peel strength evaluation of G, and a surface tack evaluationof G.

Example 4

Other than that polyvinyl acetal resin and styrene-vinyl isoprene blockcopolymer were mixed at a mass ratio of 7:3 to form the supporting resinlayer, a heat conductive sheet was manufactured as described above. Asrepresented in Table 1, Example 4 had a tensile strength evaluation ofG, a T-type peel strength evaluation of G, and a surface tack evaluationof I.

Comparative Example 2

Other than that polyvinyl acetal resin and styrene-vinyl isoprene blockcopolymer were mixed at a mass ratio of 0:10 to form the supportingresin layer, a heat conductive sheet was manufactured as describedabove. As represented in Table 1, Comparative Example 2 had a tensilestrength evaluation of F, a T-type peel strength evaluation of G, and asurface tack evaluation of F.

Comparative Example 3

Other than that polyvinyl acetal resin andstyrene-ethylene/butylene-styrene block copolymer (SEPTON V9827, KURARAYCO., LTD.) were mixed at a mass ratio of 9:1 to form the supportingresin layer, a heat conductive sheet was manufactured as describedabove. As represented in Table 1, Comparative Example 3 had a tensilestrength evaluation of G, a T-type peel strength evaluation of F, and asurface tack evaluation of G.

Comparative Example 4

Other than that polyvinyl acetal resin andstyrene-ethylene/butylene-styrene block copolymer (SEPTON V9827, KURARAYCO., LTD.) were mixed at a mass ratio of mol 7:3 to form the supportingresin layer, a heat conductive sheet was manufactured as describedabove. As represented in Table 1, Comparative Example 4 had a tensilestrength evaluation of G, a T-type peel strength evaluation of F, and asurface tack evaluation of G.

Comparative Example 5

Other than that polyvinyl acetal resin andstyrene-ethylene/butylene-styrene block copolymer (SEPTON V9827, KURARAYCO., LTD.) were mixed at a mass ratio of 3:7 to form the supportingresin layer, a heat conductive sheet was manufactured as describedabove. As represented in Table 1, Comparative Example 5 had a tensilestrength evaluation of F, a T-type peel strength evaluation of F, and asurface tack evaluation of F.

Comparative Example 6

Other than that polyvinyl acetal resin andstyrene-ethylene/butylene-styrene block copolymer (SEPTON V9827, KURARAYCO., LTD.) were mixed at a mass ratio of 0:10 to form the supportingresin layer, a heat conductive sheet was manufactured as describedabove. As represented in Table 1, Comparative Example 6 had a tensilestrength evaluation of F, a T-type peel strength evaluation of F, and asurface tack evaluation of F.

TABLE 1 T-Type Tensile Peel Surface Strength Strength Tack Eval- Eval-Eval- Supporting Resin Layer uation uation uation Comp. 1 [BX-1]:[HYBRAR5125] = 10:0 G F G Ex. 1 [BX-1]:[HYBRAR 5125] = 9:1 G I G Ex. 2[BX-1]:[HYBRAR 5125] = G G G 8.5:1.5 Ex. 3 [BX-1]:[HYBRAR 5125] = 8:2 GG G Ex. 4 [BX-1]:[HYBRAR 5125] = 7:3 G G I Comp. 2 [BX-1]:[HYBRAR 5125]= 0:10 F G F Comp. 3 [BX-1]:[SEPTON V9827] = 9:1 G F G Comp. 4[BX-1]:[SEPTON V9827] = 7:3 G F G Comp. 5 [BX-1]:[SEPTON V9827] = 3:7 FF F Comp. 6 [BX-1]:[SEPTON V9827] = F F F 0:10

As in Comparative Examples 3 to 6, in the case of adding astyrene-ethylene/butylene-styrene block copolymer having unsaturatedbonds, delamination strength between the heat conductive resin layer andthe supporting resin layer was low and excellent adhesion wasunobtainable. Furthermore, as in Comparative Example 1, in the case of asupporting resin layer comprising only polyvinyl acetal, delaminationstrength between the heat conductive resin layer and the supportingresin layer was low and excellent adhesion was unobtainable. Moreover,as in Comparative Example 2, in the case of a supporting resin layerconsisting only of styrene-vinyl isoprene block copolymer, tensilestrength was reduced in the heat conductive sheet as well as tack beingexhibited by the supporting resin layer thus degrading dry properties.

In contrast, as in Examples 1 to 4, in the case of mixing polyvinylacetal and styrene-vinyl isoprene block copolymer at mass ratios of 9:1to 7:3 to form the supporting resin layer, delamination strength betweenthe heat conductive resin layer and the supporting resin layer was highand excellent adhesion was obtainable. Furthermore, tensile strength ofthe heat conductive sheet was high as well as dry properties of thesupporting resin layer being maintainable.

REFERENCE SIGNS LIST

11 heat conductive resin layer, 12 supporting resin layer, 13release-treated film

1. A heat conductive sheet comprising: a heat conductive resin layercomprising a heat conductive acrylic resin composition; and a supportingresin layer containing a polyvinyl acetal resin and a styrene-vinylisoprene block copolymer.
 2. The heat conductive sheet according toclaim 1, wherein the polyvinyl acetal resin and the styrene-vinylisoprene block copolymer are blended at a mass ratio from 9:1 to 7:3. 3.The heat conductive sheet according to claim 1, wherein the polyvinylacetal resin is polyvinyl acetal or polyvinyl butyral.
 4. The heatconductive sheet according to claim 1, wherein the polyvinyl acetalresin has a molecular weight from 5×10⁴ to 20×10⁴.
 5. The heatconductive sheet according to claim 3, wherein the polyvinyl acetalresin has a molecular weight from 5×10⁴ to 20×10⁴.
 6. The heatconductive sheet according to claim 1, wherein the polyvinyl acetalresin has a glass transition temperature from 50 to 150° C.
 7. The heatconductive sheet according to claim 3, wherein the polyvinyl acetalresin has a glass transition temperature from 50 to 150° C.
 8. The heatconductive sheet according to claim 4, wherein the polyvinyl acetalresin has a glass transition temperature from 50 to 150° C.
 9. The heatconductive sheet according to claim 1, wherein the styrene-vinylisoprene block copolymer has a styrene content from 10% to 30%.
 10. Theheat conductive sheet according to claim 3, wherein the styrene-vinylisoprene block copolymer has a styrene content from 10% to 30%.
 11. Theheat conductive sheet according to claim 4, wherein the styrene-vinylisoprene block copolymer has a styrene content from 10% to 30%.
 12. Theheat conductive sheet according to claim 6, wherein the styrene-vinylisoprene block copolymer has a styrene content from 10% to 30%.
 13. Theheat conductive sheet according to claim 1, wherein the styrene-vinylisoprene block copolymer has a glass transition temperature from −40 to+40° C.
 14. The heat conductive sheet according to claim 3, wherein thestyrene-vinyl isoprene block copolymer has a glass transitiontemperature from −40 to +40° C.
 15. The heat conductive sheet accordingto claim 4, wherein the styrene-vinyl isoprene block copolymer has aglass transition temperature from −40 to +40° C.
 16. The heat conductivesheet according to claim 6, wherein the styrene-vinyl isoprene blockcopolymer has a glass transition temperature from −40 to +40° C.
 17. Theheat conductive sheet according to claim 9, wherein the styrene-vinylisoprene block copolymer has a glass transition temperature from −40 to+40° C.
 18. A supporting resin layer comprising: a polyvinyl acetalresin and a styrene-vinyl isoprene block copolymer.
 19. A method formanufacturing a heat conductive sheet comprising the steps of: forming asupporting resin layer by mixing and sheet-forming a polyvinyl acetalresin and a styrene-vinyl isoprene block copolymer; and forming a heatconductive resin layer by curing a heat conductive acrylic resincomposition in a state of contact with the supporting resin layer.
 20. Amethod for manufacturing a supporting sheet comprising: mixing andsheet-forming a polyvinyl acetal resin and a styrene-vinyl isopreneblock copolymer.
 21. The heat conductive sheet according to claim 2,wherein the polyvinyl acetal resin is polyvinyl acetal or polyvinylbutyral.
 22. The heat conductive sheet according to claim 2, wherein thepolyvinyl acetal resin has a molecular weight from 5×104 to 20×104. 23.The heat conductive sheet according to claim 2, wherein the polyvinylacetal resin has a glass transition temperature from 50 to 150° C. 24.The heat conductive sheet according to claim 2, wherein thestyrene-vinyl isoprene block copolymer has a styrene content from 10% to30%.
 25. The heat conductive sheet according to claim 2, wherein thestyrene-vinyl isoprene block copolymer has a glass transitiontemperature from −40 to +40° C.